The present invention relates to a method of and apparatus for insulating enclosed structures such as houses, offices and factories against heat loss.
Material used for thermal insulation is a passive agent. Incorporated in the attic of a house or in the lining of a ski jacket, the action of insulation is strictly that of retarding the flow of heat. In the end, insulation material cannot prevent heat loss; it can only slow it down. Thus if a house has been heated to 70.degree. F. and the out-of-doors is 0.degree. F., then the temperature in the house will gradually fall to match the outside temperature unless additional energy is supplied to the house.
In the U.S. building trades the effectiveness of insulation is customarily designated by an R value. The R value is a function of the thickness of the insulation material and the inverse of its thermal conductivity. it is defined with the following equation: ##EQU1## where delta T is the difference in temperature in degrees Fahrenheit between the hot side of the insulation material and its cold side
Mathematically, R cannot be defined unless there is some finite heat loss through the insulation material; otherwise a zero divisor appears in the right-hand term. The loss of heat through known insulation materials is thus implicit.
The present invention relates to a method for the efficacious application of a heat pump. Heat pumps are devices that use a low entropy energy advantageously to move high entropy heat, effecting a desired heating or cooling effect. A vast literature which includes refrigeration, heating and air conditioning exists on the subject. Heat pumps generally employ a reversible chemical or physical process which may take place isothermically. There are commercial units and theoretical devices that employ condensation and vaporization, adsorption and desorption, solution and dissolution, freezing and thawing, absorption, sublimation, hydride formation, phase change, thermoelectric transfer and other processes.
Heat pumps may be operated by electric motors, diesel engines, wind and water power, sunlight, solar-powered freon turbines, gas flames, discharged process heat, and other energy forms.
Heat pumps are attractive when fuel is expensive. Commercial vapor compression type heat pumps multiply their input energy by a factor called a coefficient of performance (COP). ##EQU2##
For the inorganic operating fluid ammonia the theoretical maximum COP is 5.55, for sulfer dioxide it is 5.67, for some of the synthetic halogenate hydrocarbons it is 6.6, in the cooling mode. For devices that operate on the water vapor/water cycle it is 3.73. The heat moved is equal to the Btu's to operate the heat pump times the COP. As a practical matter COP's of 1.8 to 4 are achievable under favorable conditions with synthetic working fluids and compression heat pumps. The most favorable condition has the source of heat at or above the temperature of the discharge. Some commercial heat pumps used for domestic hot water service use an automatic cutout when the heat source temperature falls below 45.degree. F. because their efficiency tends to be quite impaired in such an environment.
It is disclosed in U.S. Pat. No. 4,267,825, issued May 14, 1981, that the effectiveness of heat pumps is limited by the fact that they will operate and provide a COP of 2 to 4 only when the outdoor air temperature is greater then between about 45.degree. to 50.degree. F. At lower temperatures, the COP is reduced until, at a temperature of about 10.degree. to 20.degree. F., the heat pump can no longer effectively draw any heat from the ambient outside air.
The present invention relates to means for absorbing and utilizing solar irradiation. Even in the coldest parts of the United States, substantial solar energy falls upon our structures. In a paper entitled "A Rational Procedure for Predicting the Long-Term Performance of Flat-Plate Solar-Energy Collectors", in Vol. 7, No. 2 edition of Solar Energy (1963), Liu and Jordan give values of monthly average daily total radiation received on a horizontal surface after penetrating average local atmospheric conditions. They offer data on average daily total Btu's per square foot. Some of their data for St. Cloud, Minn., are quoted below. Calculated from these data are the energy equivalent of this radiation in gallons of fuel oil for the month for a house assumed to be a 30-foot square lying on the ground, using the nominal 132,000 Btu's per gallon energy content for the fuel oil. A three-dimensional house of course presents greater collection surface than a full-scale floor plan, so there is some understatement of the calculated results.
______________________________________ Avg. Btu/day/ sq. ft Gal./Mo. ______________________________________ October 890.4 191 November 545.4 111 December 463.1 97 January 632.8 133 February 976.7 186 March 1383.0 292 ______________________________________
For the typical house, most of this energy falls on the roof. Much of it is reflected, reradiated, blown away in the wind, or convected to the sky in a thermal. Some of the remainder is conducted to the attic, where standard practice has it dissipated by convection through vents to the outside. The purpose of the ventilation is to evacuate water vapor which might otherwise condense in the insulation and to cool the roof so that snow cannot melt, later to form ice dams that can be quite injurious to a house. Thus the example home in central Minnesota rejects by design the energy equivalent of a thousand gallons of fuel oil provided naturally each heating season. Investigation of means to capture and utilize some of this lost energy is intense.
Above the atmosphere of the earth solar energy density has been found to average 1353 watts per square meter. The Stefan-Boltzmann Law predicts the temperature to which a perfect absorber will be heated by radiation from the sun: EQU E=kT.sup.4 (Eq. 3)
E being watts per square meter, k being 5.603/100,000,000 and T being the temperature in degrees Kelvin. Substituting in Stefan-Boltzmann gives the temperature at 394.degree. K., equal to 121.degree. C. or 250.degree. F. This is an equilibrium temperature. Cooled below this equilibrium, a perfectly black collector surface absorbs the mostly high-energy, short-wavelength radiation, much of it visible light, emanating from the sun. The collector heats up. Above this equilibrium temperature the collector radiates more energy than it takes in. It sends low-energy, long-wavelength, invisible radiation out into the 4.degree. K. heat sink of the far reaches of the universe. It cools down to 250.degree. F.
Down on earth the same process takes place, but the solar radiation is attenuated by the atmosphere. On earth, maximum energy density is about 1000 watts per square meter. Thus on a clear day a perfect absorber would equilibriate at a theoretical maximum of about 198.5.degree. F. In practice the Stefan-Boltzmann equilibrium is hardly approached. Even under unobscured insolation, a black piece of tin could reach 25.degree. to 50.degree. F. above ambient at best, useless except perhaps to fry an egg on a hot day. Heat is lost to the atmosphere at a rate of 2 to 10 Btu's per square foot per degree of temperature F. that the collector attains above ambient.
Thus there are two problems. There is the problem of reradiative heat loss that exists even in outer space. There is the problem in the earthly environment of heat loss to the surroundings.
The most widely known approach to the problem of reradiation is to employ the "greenhouse effect." Many glasses and plastics have a transparency to the sun of 98% under laboratory conditions and up to 90% in field conditions in stationary solar collectors. They are however opaque of dark to the long wavelength infrared that is reradiated from the collector's absorber. By covering the collector with one or two transparent sheets, the maker of the solar collector lets the sunlight go in but hinders the absorbed energy from leaving.
An additional or alternative measure is to fabricate the collector with a selective surface: an ultra-thin layer of a material which is good at absorbing sunlight is coated on a substrate such as shiny metal which is known to be poor at radiating long-wave energy. One successful approach involves nickel plating the absorber and then overcoating the nickel with an electro-deposition of chromium oxide. Selective surfaces that absorb 95% of the solar energy but emit only 10% of long wave energy are known in commerce.
The greenhouse has the beneficial side effect of blanketing the collector in a layer of still air, thus reducing the conductance of its heat to the surroundings. It ameliorates both problems with a single mechanism.
One commercial device uses a selective absorber surface encased within a partial vacuum held by a glass tube. Its heat loss may be as small as 0.2 Btu per square foot per degree F temperature difference, almost an order of magnitude less than a collector incorporating a single glass cover--and 50 times less heat loss than an unadorned flat black plate under average conditions on earth.
The technology has proven fruitful, especially under optimum conditions. Positioned, say, in a desert with dry clear air and a temperature in the sun of 130.degree. F., commercially available flat solar collectors are capable of delivering upwards of 70% of the 300 Btu's per square foot per hour provided by sunlight. Output temperatures may suffice to char wood.
Efficiency drops, however, when the sunlight is less intense and the ambient temperature lowers. Even on the desert the collector that does so well in the heat may drop to 27% efficiency if the temperature lessens to 30.degree. F. and a haze attenuates the sunlight by half. Then instead of delivering 210 Btu's per hour, it offers 40.
In the current state of the art, solar collectors fall to zero efficiency when it is quite cold and the sun is obscured or reaches the collector surface at a small angle of incidence. At sub-zero F temperatures commercial units may actually be counterproductive, consuming more operating energy than they wrest from the sunlight. When produced by a reliable manufacturer using sound technology, competently applied and installed, solar collectors of the flat-plate type are, despite the foregoing limitations, operable and useful. They are not, however, economical for Americans in general. At this writing, the bottom line comes out in favor of fossil fuel for space and water heating because of high capital and interest costs. The most promising application in the northern part of the United States is partial load hot water heating (a modest-size unit's assuming a less than total responsibility for all a family's hot water, i.e. with a backup). The equipment may be productive almost year 'round and is of use when operating conditions are most ripe--during hot, sunny days. Recent local analysis, however, revealed this application to suffer in comparison with the most expensive alternative, namely electrical resistance water heating. For consumers of high-rate REA-supplied resistance hot water heat, a 40%+Federal and state tax credit on the full capital cost makes partial load hot water heating economically efficient if the consumer has sufficient tax liability to absorb the tax credit.
Passive solar energy utilization is more economically attractive in general. Passive systems can be simple, indeed. Where there exists a heavy heating load and sunny winter days, planning fenestration for sunny exposures gains considerable advantage with little or no attached marginal cost. There are drawbacks and objections to even such simple and sensible measures. They may result in quite unconventional looking structures, which some find distasteful. Others may fear this appearance places their housing investment at added market risk. While many passive solar energy techniques can be employed virtually without cost when incorporated during the planning stage, they may be unfeasible in retrofit. The replacement rate of buildings is so low that if all new construction were to exploit fully passive solar energy technology, a pronounced net effect would be slow to appear.
It is an object of the present invention to provide an apparatus and method to prevent the conduction of heat from the interior of an enclosed structure to its exterior.
It is a further object of the invention to provide a method of installing a heat pump so that its coefficient of operation is independent of temperature in the winter and so that it is operative and efficient at temperatures below the melting temperature of ice.
It is a further object of the invention to provide an apparatus to capture and utilize energy contained in the sunlight falling upon an enclosed structure even at sub-zero temperatures.
It is a further object of the invention to provide an apparatus and method to retrofit existing enclosed structures for hyperinsulation and solar heating.
It is a further object of the invention to provide a method and apparatus to prevent the conduction of heat from the interior of a structure to its exterior through ordinarily difficult places to insulate, including fenestration.